This invention relates to high temperature aluminosilicate glasses containing CaO and BaO in the proper proportions so that the glass reliably provides a hermetic seal to molybdenum metals in critical applications such as commonly employed for incandescent lamps being operated at elevated temperatures of 500.degree. C and higher. More particularly, certain electric lamp designs require a transparent envelope material capable of withstanding very high temperatures wherein said envelope is also sealed hermetically to electrical in-leads. Among these lamp types are regenative cycle, halogen containing lamps, and high intensity discharge lamps. Fused quartz was the original lamp envelope material used for transparent hermetically sealed envelopes in such lamps because of its high temperature capabilities along with Vycor glass, but both glasses proved expensive and difficult to work with at the extremely high working temperatures involved.
More recently, two high temperature glasses were developed in the aluminosilicate system. These glasses are used in iodine cycle incandescent lamps by reason of being alkali metal oxide free to prevent darkening of the glass by formation of alkali metal halide crystals on the interior wall of the lamp envelope during operation. Both glasses consist essentially of SiO.sub.2, Al.sub.2 O.sub.3, BaO, and CaO with an optional B.sub.2 O.sub.3 content in small amounts along with still other metal oxides being employed to adjust the thermal coefficient of expansion for a proper match with the particular metal body in the glass-to-metal seal. One glass employs in percentages by weight 57.3 SiO.sub.2, 16.2 Al.sub.2 O.sub.3, 9.4 CaO, 5.6 MgO, 0.036 K.sub.2 0, 0.11 Na.sub.2 O, 4.3 B.sub.2 O.sub.3, 0.2 ZrO.sub.2, 0.03 Li.sub.2 O, 0.02 Rb.sub.2 O, and 6.3 BaO along with residual oxide fluxes and exhibits a linear thermal coefficient of expansion of approximately 44.3 .times. 10.sup.-7 cm/cm/0.degree. C in the 0-300.degree. C temperature range. The other glass employs in percentages by weight 65.5 SiO.sub.2, 18.8 Al.sub.2 O.sub.3, 7.2 CaO, 0.01 K.sub.2 0, 0.14 Na.sub.2 O, 0.09 ZrO.sub.2, and 8.3 BaO and provides a 35.7 .times. 10.sup.-7 cm/cm/.degree. C linear thermal expansion coefficient over the aforementioned temperature range. A principal compositional difference between said glasses is the presence of B.sub.2 O.sub.3 in the higher expansion glass which undesirably lowers the softening point and strain point for glass-to-metal seal applications.
An improved aluminosilicate glass for direct hermetic sealing to molybdenum in-leads is thereby still desired with a thermal expansion in the approximate range 42-48 .times. 10.sup.-7 cm/cm/.degree. C so that either flat or round leads can be used without introducing significant residual strain and with said glass seal permitting higher temperature operation as well as other advantages. In achieving this objective for molybdenum in-leads, it is generally required that very thin flat foils be used with the conventional aluminosilicate glasses, and the use of such foils has limited current carrying ability of the regenerative cycle incandescent lamps and also proved expensive in producing said lamps. It would also be desirable to have glasses with higher applicability temperatures which do not require working temperatures as high as the available aluminosilicate glasses. The strain point of the glass composition is significant for such lamp applications since the glass envelope must withstand temperatures in excess of 500.degree. and usually in the range 500.degree.-700.degree. C without either distortion or failure. The glasses are generally formed into lamp envelopes from tubing at temperatures above their softening point. The liquidus temperature of the glass composition also has significance during glass manufacture and is the minimum temperature at which the glass can be formed without precipitating undesired crystals over a period of time. In considering all of the above indicated physical properties of a desirable aluminosilicate glass for this type hermetic sealing application, therefore, it becomes desirable to have as high a strain point as possible along with a liquidus temperature as low as possible.